I think pg's solution is to ignore what you don't like:

Please don't submit comments complaining that a submission is inappropriate for the site. http://ycombinator.com/newsguidelines.html

He's talking about "inappropriate for the site" submissions, not "poor quality" submissions - but I think the same reasoning applies.

Perhaps the solution isn't a downmod button, but for the community to change its response? To ignore bad submissions instead of commenting on how bad they are. Commenting just draws attention to the submission and contributes to the problem.

I think there's a deeper philosophy here, that it would be worth hearing more about...